今天遇到一个问题，需要在API请求结束时，释放数据库链接，避免连接池被爆掉。
按照以往的经验，需要实现IHttpModule，具体不展开了。
但是实现了IHttpModule后，还得去web.config中增加配置，这有点麻烦了，就想有没有简单的办法。
其实是有的，就是在Global.asax.cs里面定义并实现 Application_EndRequest 方法，在这个方法里面去释放数据库连接即可，经过测试，确实能达到效果。
但是，为什么方法名必须是Application_EndRequest ？在这之前真不知道为什么，只知道baidu上是这么说的，也能达到效果。
还好我有一点好奇心，想搞清楚是怎么回事情，就把net framework的源码拉下来(其实源代码在电脑里面已经躺了N年了) 分析了一下，以下是分析结果。
省略掉前面N个调用
第一个需要关注的是 HttpApplicationFactory.cs
从名字就知道，这是HttpApplication的工厂类，大家看看Gloabal.asax.cs 里面，是不是这样定义的
public class MvcApplication : System.Web.HttpApplication
{
.....
}
两者结合起来看，可以推测，HttpApplicationFactory 是用来获取 MvcApplication 实例的，实际情况也是如此 上代码（来自HttpApplicationFactory）
internal class HttpApplicationFactory{
internal const string applicationFileName = "global.asax"; //看到这里，就知道为什么入口文件是global.asax了，因为这里定义死了
...
private void EnsureInited() {
if (!_inited) {
lock (this) {
if (!_inited) {
Init();
_inited = true;
}
}
}
}
private void Init() {
if (_customApplication != null)
return;
try {
try {
_appFilename = GetApplicationFile();
CompileApplication();
}
finally {
// Always set up global.asax file change notification, even if compilation
// failed. This way, if the problem is fixed, the appdomain will be restarted.
SetupChangesMonitor();
}
}
catch { // Protect against exception filters
throw;
}
}
private void CompileApplication() {
// Get the Application Type and AppState from the global file
_theApplicationType = BuildManager.GetGlobalAsaxType();
BuildResultCompiledGlobalAsaxType result = BuildManager.GetGlobalAsaxBuildResult();
if (result != null) {
// Even if global.asax was already compiled, we need to get the collections
// of application and session objects, since they are not persisted when
// global.asax is compiled. Ideally, they would be, but since <object> tags
// are only there for ASP compat, it's not worth the trouble.
// Note that we only do this is the rare case where we know global.asax contains
// <object> tags, to avoid always paying the price (VSWhidbey 453101)
if (result.HasAppOrSessionObjects) {
GetAppStateByParsingGlobalAsax();
}
// Remember file dependencies
_fileDependencies = result.VirtualPathDependencies;
}
if (_state == null) {
_state = new HttpApplicationState();
}
// Prepare to hookup event handlers via reflection
ReflectOnApplicationType();
}
private void ReflectOnApplicationType() {
ArrayList handlers = new ArrayList();
MethodInfo[] methods;
Debug.Trace("PipelineRuntime", "ReflectOnApplicationType");
// get this class methods
methods = _theApplicationType.GetMethods(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static);
foreach (MethodInfo m in methods) {
if (ReflectOnMethodInfoIfItLooksLikeEventHandler(m))
handlers.Add(m);
}
// get base class private methods (GetMethods would not return those)
Type baseType = _theApplicationType.BaseType;
if (baseType != null && baseType != typeof(HttpApplication)) {
methods = baseType.GetMethods(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static);
foreach (MethodInfo m in methods) {
if (m.IsPrivate && ReflectOnMethodInfoIfItLooksLikeEventHandler(m))
handlers.Add(m);
}
}
// remember as an array
_eventHandlerMethods = new MethodInfo[handlers.Count];
for (int i = 0; i < _eventHandlerMethods.Length; i++)
_eventHandlerMethods[i] = (MethodInfo)handlers[i];
}
private bool ReflectOnMethodInfoIfItLooksLikeEventHandler(MethodInfo m) {
if (m.ReturnType != typeof(void))
return false;
// has to have either no args or two args (object, eventargs)
ParameterInfo[] parameters = m.GetParameters();
switch (parameters.Length) {
case 0:
// ok
break;
case 2:
// param 0 must be object
if (parameters[0].ParameterType != typeof(System.Object))
return false;
// param 1 must be eventargs
if (parameters[1].ParameterType != typeof(System.EventArgs) &&
!parameters[1].ParameterType.IsSubclassOf(typeof(System.EventArgs)))
return false;
// ok
break;
default:
return false;
}
// check the name (has to have _ not as first or last char)
String name = m.Name;
int j = name.IndexOf('_');
if (j <= 0 || j > name.Length-1)
return false;
// special pseudo-events
if (StringUtil.EqualsIgnoreCase(name, "Application_OnStart") ||
StringUtil.EqualsIgnoreCase(name, "Application_Start")) {
_onStartMethod = m;
_onStartParamCount = parameters.Length;
}
else if (StringUtil.EqualsIgnoreCase(name, "Application_OnEnd") ||
StringUtil.EqualsIgnoreCase(name, "Application_End")) {
_onEndMethod = m;
_onEndParamCount = parameters.Length;
}
else if (StringUtil.EqualsIgnoreCase(name, "Session_OnEnd") ||
StringUtil.EqualsIgnoreCase(name, "Session_End")) {
_sessionOnEndMethod = m;
_sessionOnEndParamCount = parameters.Length;
}
return true;
}
}
上面代码调用链路是EnsureInited->Init->CompileApplication->ReflectOnApplicationType->ReflectOnMethodInfoIfItLooksLikeEventHandler ,核心作用是：将MvcApplication中，方法名包含下划线、方法参数为空或者有2个参数(第一个参数的类型是Object,第二个参数的类型是EventArgs) 的方法加入到_eventHandlerMethods 中
那么事件是怎么绑定的呢？继续上代码
internal class HttpApplicationFactory{
......
using (new ApplicationImpersonationContext()) {
app.InitInternal(context, _state, _eventHandlerMethods);
}
......
......
using (new ApplicationImpersonationContext()) {
app.InitInternal(context, _state, _eventHandlerMethods);
}
......
}
// HttpApplication.cs
public class HttpApplication{
internal void InitSpecial(HttpApplicationState state, MethodInfo[] handlers, IntPtr appContext, HttpContext context) {
.....
if (handlers != null) {
HookupEventHandlersForApplicationAndModules(handlers);
}
.....
}
internal void InitInternal(HttpContext context, HttpApplicationState state, MethodInfo[] handlers) {
.....
if (handlers != null)
HookupEventHandlersForApplicationAndModules(handlers);
.....
}
private void HookupEventHandlersForApplicationAndModules(MethodInfo[] handlers) {
_currentModuleCollectionKey = HttpApplicationFactory.applicationFileName;
if(null == _pipelineEventMasks) {
Dictionary<string, RequestNotification> dict = new Dictionary<string, RequestNotification>();
BuildEventMaskDictionary(dict);
if(null == _pipelineEventMasks) {
_pipelineEventMasks = dict;
}
}
for (int i = 0; i < handlers.Length; i++) {
MethodInfo appMethod = handlers[i];
String appMethodName = appMethod.Name;
int namePosIndex = appMethodName.IndexOf('_');
String targetName = appMethodName.Substring(0, namePosIndex);
// Find target for method
Object target = null;
if (StringUtil.EqualsIgnoreCase(targetName, "Application"))
target = this;
else if (_moduleCollection != null)
target = _moduleCollection[targetName];
if (target == null)
continue;
// Find event on the module type
Type targetType = target.GetType();
EventDescriptorCollection events = TypeDescriptor.GetEvents(targetType);
string eventName = appMethodName.Substring(namePosIndex+1);
EventDescriptor foundEvent = events.Find(eventName, true);
if (foundEvent == null
&& StringUtil.EqualsIgnoreCase(eventName.Substring(0, 2), "on")) {
eventName = eventName.Substring(2);
foundEvent = events.Find(eventName, true);
}
MethodInfo addMethod = null;
if (foundEvent != null) {
EventInfo reflectionEvent = targetType.GetEvent(foundEvent.Name);
Debug.Assert(reflectionEvent != null);
if (reflectionEvent != null) {
addMethod = reflectionEvent.GetAddMethod();
}
}
if (addMethod == null)
continue;
ParameterInfo[] addMethodParams = addMethod.GetParameters();
if (addMethodParams.Length != 1)
continue;
// Create the delegate from app method to pass to AddXXX(handler) method
Delegate handlerDelegate = null;
ParameterInfo[] appMethodParams = appMethod.GetParameters();
if (appMethodParams.Length == 0) {
// If the app method doesn't have arguments --
// -- hookup via intermidiate handler
// only can do it for EventHandler, not strongly typed
if (addMethodParams[0].ParameterType != typeof(System.EventHandler))
continue;
ArglessEventHandlerProxy proxy = new ArglessEventHandlerProxy(this, appMethod);
handlerDelegate = proxy.Handler;
}
else {
// Hookup directly to the app methods hoping all types match
try {
handlerDelegate = Delegate.CreateDelegate(addMethodParams[0].ParameterType, this, appMethodName);
}
catch {
// some type mismatch
continue;
}
}
// Call the AddXXX() to hook up the delegate
try {
addMethod.Invoke(target, new Object[1]{handlerDelegate});
}
catch {
if (HttpRuntime.UseIntegratedPipeline) {
throw;
}
}
if (eventName != null) {
if (_pipelineEventMasks.ContainsKey(eventName)) {
if (!StringUtil.StringStartsWith(eventName, "Post")) {
_appRequestNotifications |= _pipelineEventMasks[eventName];
}
else {
_appPostNotifications |= _pipelineEventMasks[eventName];
}
}
}
}
}
}
核心方法：HookupEventHandlersForApplicationAndModules，其作用就是将前面获取到的method与HttpApplication的Event进行绑定(前提是方法名是以Application_开头的)，
后面就是向IIS注册事件通知了，由于看不到IIS源码，具体怎么做的就不知道了。
最后安利一下，还是用net core吧，更加清晰、直观，谁用谁知道。